Equalization of power level in alpha signal transmitting system



D. MITCHELL April 12, 1932.

Filed Dec. 29, 1928 ATTORN EY I outrun stares;

Patented Apr. 12, 1932' PATENT OFFICE DOBEN MITCHELL, OF -NEWYORK, N.Y., ASSIGNOIR TO AMERICAN TELEPHONE ,AND TELEGRAPH VC OMEANY,ACORPORATION' OF NEW, YORK EQUALIZATION, or rownn nnvnn IN A SIGNALTRANSMITTING SYSTEM 2 Application filed December 29; 1928. Serial No.329,203.

mitting system to efiect adjustment of a condenser charge applied so astocorrect a tendency for the signals to depart from a deslred powerlevel at that point. Another object indicated injthe appended claims.

of my invention is to provide for keeping the power level adjuster to adesired standardvalue in a telephone j transmission system, In oneaspect, a device-accordmg to my in- Venti'onoperates by virtue of the)power in the, signaling Currents andthereby adjust-- ment of a suitablyconnected condenser is efg fected to hold the power levelat a desiredvalue. My invention may be practiced by means ofthree-electrodevacuum'tuhes with- ;out the use of relays having make-and-breakcontacts. V

p The accompanying drawing is adiagram- V maticrepresentation showing anexample of the practice of'my invention. It'will be understood thatthefollowing specification relat-es principally to this particular exampleofthe invention and that its scope will be "Referring to the drawing,this shows a four-wire telephone system which may be'a link connectingwith a two-way radio telephone system. Transmission from west to east isover the lineL through the amplifier A' and on over the 7 line LTransmission from east to west is similarly through the line L'amplifier A and line ,L' The amplifiers A and A constitute a four-wirerepeater.

V1 and V afford a shuntp athacross theline The system shown in thedrawing tests the power levelon the output side ofthe ampli iier A and,shunts more or less of theenergyj passing from L to L; soas to correctfor a,

departure from proper standard value in the energy level at the outputof the amplr:

fier A. y c 1 i It will be seen that the two vacuum tubes L and'theadjustment referred to is :made by varying thedynamic impedance of theshunt'path through these two tubes, as will be'explained farther alongin this specification. For reasons whichwill appear, a reverseadgustment is made in a correspondv ing shunt in the line L' L Thefour-wire system shown in the drawing is arranged so that transmissionin one direction cuts oii' the system for transmission in the oppositedirection, and vice versa. Thus objectionable echo effects are preventedNormally, the line LI L is short circuited at 21. so that thesystem'stands in read ness only'for transmission from eastto. West.\Nhen'transmlsslon from east to west occurs, a little of the voicecurrent energy goes by a. branch circuit to the amplifying d'etectorADwhich operates a relay to shortcircuitthe line L L at22. The delaycircuit D0 gives assurance that p the relay 22 will, be operated beforeenergy fromeast to west can get around and operate the amplifyingdetector AD a path for suchenergy might be afforded by some degree ofunbalance at the three-winding transformer or hybrid coil H.

When voicecurrents'are put on the systenr from west tofeast, theyjoperatethe amplifying detector AD andoperaterbothrelays at21 and 23, thusshort-oircuiting theline from east to west at 23, and opening, the

short circuit from west to east at '21. The

delay circuit DC, gives assurancethat the short-circuit at 21 will beopened beforethe voice currents reach'this point on their way from westto east on the line L1 v Proceedingfrom west to east along the line L1L2, the repeating coil RC gives a high ratio step up and the repeatingcoil RO gives an equal ratio step down. Hence, between these two coilsthe circuit is of high imped ance. Normally, direct currents'areflowing: inthe plate circuits of the two vacuum'tuhes V and Vi, and thepath from, plate to fila ment in V and'from filament to plate in V is ahigh impedance shunt path across the two sides of theline'between thetwo repeating coils RC and RC ,The shunt impedance here in view is adynamic impedance and is the ratio of'voltage increment to current in-Vcrement. This ratio is the slope on thevoltamperecharacteristic of thetubes Vyand V5,-

The point of this characteristic that is in volved is determined by thevoltage on the grids of the tubes V and V and this is determined by thecharge on the condenser C Normally, there will be no charge on thecondenser C This condenser might receive a charge in the plate circuitof either tube V or V but the grid of the tube V is at such a negativevalue that no condenser char ing current willfiow in the plate circuitof Also, as will be shown presently, there is a potential drop in theresistance R that puts a sufiicient negative voltage on the grid of thetube V to prevent ny charging of the condenser C in the plate circuit ofthat tube.

Normally, a considerable current is flowing in the plate circuit of thetube V; and this gives the potential drop in the resistance R that wasmentioned above. Although the grid battery for the tube V; is negative,its electromotive force is counter-balanced in the grid circuit by anopposite potential drop along the resistance R due to current flowing inthe plate circuit of the tube V This is the normal condition for theapparatus mentioned in the foregoing description. Now, suppose voicecurrents come from the west over the line L L These currents areamplified in the tube V and affect the detector tube V Due to theblocking condenser C and resistance R in the grid circuit of the tube Vtheir only action is to reduce the plate current of the tube V Thisallows the condenser C to discharge and reduces the potenial dropthrough the resistance R, thus permittting the grid circuit battery ofthe tube V to become effective.

The tubes V and V and the transformers between the line L and the tube Vand between the two tubes are so chosen that the discharge of thecondenser C can be just brought about by the peaks of speech currentfrom the weakest voice for which the system is designed to operate, butnot by the noise current-s normally present. Stronger voice waves canproduce no greater change than this because the space current of tube Vcannot become negative. The change of potential of the grid of thevacuum tube V;,, due to the removal of the drop in the resistance R dueto the space current of tube V thus occurs suddenly on the arrival ofspeech waves but is substantially independent of the magnitude of thesewaves.

But the voice currents from amplifier A to line L go through therepeating coil RC into the grid circuit of the tube V and affect thetube V as a detector and counteract the effect ofthe negative battery inthe grid circuit mentioned in the preceding paragraph.

Thus, if the power level is correct at the output from the amplifier A,and if adjustments have been made accordingly, re-

moval of potential drop across the resistance R will be counter-balancedin the grid circuit of the tube V by the impulses received through therepeating coil RC and the current in the plate circuit of the tube Vwill remain unchanged. Thus, there will be no tendency for the condenserC to become charged by current in the plate circuit of the tube V Theadjustment of the plate circuit battery of the tube V, is such thatcurrents in the repeating coil RC due to a normal power level at theoutput of amplifier A, will not be sufficient to overcome the negativebias on the grid of the tube V and therefore the condenser C will nottend to charge by current in the plate circuit of the tube V Now,suppose the power level is too low at the output of amplifier A, as, forexample,

if the speaker at the station on the west 5 speaks in too low a voice.The result will be a subnormal transfer of energy into the grid circuitof the tube V through the repeating coil RC accordingly, the gridbattery for that tube will put higher negative potential on the grid,and the plate circuit current in the resistance R will be decreased,thus cutting down potential in the grid of the tube V and permitting acharging current for the condenser C to flow in its plate circuit. Thecondenser C will charge slowly. This will make the grids of the tubes Vand V: more negative and increase the dynamic impedance in the shuntbranch through the two tubes V and V 2 in series. Hence, more of thespeech current energy on the line Will be transmitted through therepeating coil RC into the amplifier A, and the tendency of a low powerlevel on the output side of this amplifier A will be corrected.

On the other hand, suppose the power level on the output side of theamplifier A is too high, this will have no effect in the plate circuitof the tube V but will operate on the tube V as a detector through therepeating coil RC4, and accordingly a current will flow in the platecircuit of the tube V and char e the condenser C slowly in a directionto taEe negative potential off the grids of the tubes V and V This willlower the impedance of the shunt through the tubes V and V- and hencemore of the voice current energy coming in over the line L will beshunted and less will go through the repeating coil RC into theamplifier A, and thus a compensatory adjustment will be effected on theoutput side of the amplifier A.

' The line from east to west, from L through the amplifier A to L has ahigh impedance section like the line the other way, with two tubes V'and V corresponding to V and V In the line L -L we have seen thatadjustment is effected by varying the potential on the grids of thetubes V and V It will be seen at once from the the system works asfollows; If the person at the west end stops talking,the shunt at I 23willopen after thelapseof a certain hangover interval, Suppose that atonce voice currents come from east. Due to the likelihood of some degreeof unbalance at H, some of the energy of these voice currents will getinto the line L and with high over-all amplification from L to L therewould be v a tendency to diversion of energy through AD andanundesirable operation ofrelays '21 and 23. But when thereis highover-all amplification from Li to L there will-be low over-allamplification from I1 to 1/ and thus the tendency to cause such falseoperation will be'counteracted. Y a

It willbe understood that all impedance elements, batteriesjetc. areadjustable; certain of these for which 'adjustment is most importanthave the adjustability indicated diagrammatically in the drawing.

Itmay be noticed that with suitable adjustment of the apparatus shown inthe drawing no action'will take place whencorrect volume is beingdelivered at the output from the amplifier A. The proper adjustmentsin-c volve certain time constants and the relative ains in the system.According to these ad justmnts, when any speech whatever is goingthrough the system from west to east the tube V will be blocked, thustending to interrupt the plate current in "the tube V and permitting thecharging current to flow in the plate circuit of the tube V to the condenser C However, the condition for correct volume is that the speechcurrent impulses operating in the grid circuit of the tube V through therepeatingcoil RC shall be just strong enough to overcome the grid biasso that the current in the plate circuit 7 of the tube V shallbemaintained and keep a potentialdrop through theresistance R and thusprevent the flow of a charging cur rent for the condenser C in the platecircuit speech currents of correct volume.

7 As to the time actions, the measure ofthe capacity of the condenserCgdeterminesthe speed with which regulation accomplished If-thevolumedeparts from standard bya large amount theinitial regulation "will takeplace faster than if the volume departs 'to a less degree.Thisadvantageous feature or operation is due to the fact that theimpedance through which the condenser will charge will be less if thevolume departs from I the standard by a'large amount than if it departsby a small amount. The system shown in the drawingis back acting; thismeans that regulation is accomplished bythe use of'energytaken from thepoint at which standard, volumes isdesired, that is,; at the output;from the amplifier A,and adjustment to preserve constancy of volumeoccurs when the volume at this pointtends to change. I a l 1 The systemshown in the drawing is also full neutral?,i,that is,yit doesnot-regulate unless speech currents (or currents due to interference,for example) are entering the syste'm, butthev system stays at the gainlast set until such currents again enter the sy tern, or until thecharge on condener G has slowly leakedawayi -c However, by changing thegrid bias on either of the tubesV or V; it is easy to make thesetubesdraw a slight plate currentunder normal conditions and-thus causethe device to creep back to a predetermined setting Forthis purpose theprincipal adjustments will be madeat the resistance R and attlie c gridbattery of the tube V4. In this way the.

system can bemade to assume a desircdad- V j ustment'of normal gainaftertheend of any 1; In thetransmission ofjsignals by .elec tric currents,the method of adjusting over H all ga n at an amplifier'whlch consistsin i shunting part ofthe current adjacent to the amplifier through avacuum tube, varyingits cgrid potential to adjust the proportion of thecurrent shunted, through the tube and cordance with the output from saidamplifier.

2.1111 the transmission of signals by electric currentsover apairofconductorsj the method of ad ustmgloss or gain wh-1chcon efi'ecting thevariation of said grid potential; by varying the .chargeona condenser inact-t sists, instepping the voltage up over at short stretch of theconductorpair and then down,g p and in thisshort stretch shunting partof the, a cui'irent through a vacuum tube, andivarying itsimpedancebyqjvarying its grid potential:

ed (to thatctransinitte id'.

Qto'adjust the proportion of the current shunt,-

tials in opposite directionsfor the two shunts to increase or'decreasetheimpedance of the shunt andslmultaneously decrease orincrease;

the impedance of the'other shunti j 1 .;.;Inafour wiretWo-waysignaltransmit-r ting system, means for cutting off transmission one waywhile it occurs the other way, amplifiers respectively for the two ways,vacuum tube shunts respectively on the two pairs of the four-wiresystem, and means to increase the grid potential in one such shunt andsimultaneously decrease it in the other so that as the over-allamplification for one pair is increased or decreased, the over-allamplification for the other pair will be simultaneously decreased orincreased.

5. A conductor pair for transmitting signals, step-up and step-downtransformers to increase the impedance between them, a vacuum tube shuntacross the pair between these transformers and means to vary the gridpotential in this shunt to adjust the shunt impedance and thereby adjustthe proportion of current that is shunted to the proportion transmitted.

6. A signal transmitting circuit comprising a pair of conductors, avacum tube shunt across the conductors, means to test the amplitude ofthe signaling current on the output side of said shunt, and meansoperated thereby to adjust the grid potential of the vacuum tube tocompensate for changes of power level acording to the test.

7. In a four-wire two-way signal transmitting system, vacuum tube shuntsacross each pair of wires, means to test the power level on the outputside of one such shunt, and means to adjust the grid potentials of thevacuum tubes to compensate a tendency to change the power level for thepair on which it is tested and to effect an opposite change in the powerlevel in the other pair.

8. In a four-wire two-Way signal transmitting system, means to cut offtransmission one way when it is in course the other way comprisingamplifiers respective to the two ways of transmission, two vacuum tubeshunts, one associated with each amplifier, means to test the powerlevel on the output side of one amplifier, and means to adjust the gridpotentials in said "acuum tubes shunts thereby so as to keep the powerlevel at the point of test substantially constant butto vary the powerlevel by opposite adjustment in the other pair of the system.

9. In a four-wire two-way signal transmitting system, a voice operateddetector-amplifier set to block transmission one way on one pair ofwires when signal transmission the other way on the other pairs is inprogress, vacuum tube shunts for each pair of wires and means to changethe grid potentials in oppo site directions for the two shunts toincrease or decrease the impedance of one shunt and simultaneouslydecrease or increase the impedance of the other shunt.

In testimony whereof, I have signed my name to this specification this28th day of December, 1928.

DOREN MITCHELL.

